1. Field of the Invention
The present invention relates to a time-sequential colour projector. Applications of such projectors include video and data projectors used to project images from electronic data sources. Such projectors include front and rear projectors, projectors for cinema and home entertainment, projectors for head-up displays in vehicles, projectors for business and conference applications and miniature portable projectors.
2. Description of the Related Art
Electronic projectors typically make use of one or more ‘light valves’, which are typically planar devices divided into pixels. The fraction of incident light which each pixel transmits or reflects can be controlled independently. A projector also contains an optical system for illuminating such a light valve, electronics for controlling the valve, and an optical system for projecting an image displayed by the light valve on a screen.
Three types of light valves are currently used in known projectors, namely transmissive liquid crystal panels, reflective liquid crystal panels, and micromirror devices. Each of these devices is monochrome: the fraction of light each pixel reflects or transmits is controlled but the colour is not.
A colour projector can be constructed using monochrome light valves in a number of ways. Most commercially available projectors use either the three-panel system or the time-sequential colour system.
In a three-panel projector, red light is sent through one panel, green light through another, and blue light through a third panel. Images of the three panels are superimposed using combining optics such as a dichroic prism.
In a time-sequential colour projector, a single light valve is used. An image is displayed and the valve is illuminated first with red light; then the image displayed is changed, and the valve is illuminated with green light; then the image is changed again and the valve is illuminated with blue light. This sequence is repeated rapidly enough for the viewer's eye to fuse the three single-colour projected images into a single, colour image.
The time-sequential colour system has the following disadvantages. Flicker or colour artefacts may be visible. A fast light valve (such as a micromirror device) is necessary and these are typically more expensive than relatively slow light valves such as transmissive liquid crystal panels. If a white light source is used, then at any time, less than one-third of the light output can be used. For example, when the red image is displayed, green and blue light from the light source must be discarded. This means that the optical efficiency of the projector is low.
The three-panel system has the disadvantage that three panels and colour separating and combining optics are required, increasing the cost of the system. Also, the images from the three panels must be made to converge with high precision, thus complicating the manufacturing process.
These projector architectures are well known and are disclosed in detail by Stupp and Brennesholtz in the book Projection displays (Wiley 1999).
A known system which overcomes some of the shortcomings of the three-panel and time-sequential designs is disclosed in U.S. Pat. No. 5,161,042. A lamp emits white light. The red, green and blue components are reflected to slightly different angles by three colour-selective (dichroic) mirrors. The light valve is a transmissive panel. An array of microlenses is placed close to the panel, on the side facing the light source, so that the pixels of the panel are in the focal plane of the microlenses.
The lenses are arranged so that, because of the different directions of travel of the red, green and blue beams, the different colours are sent to different pixels in the display panel. This is illustrated in FIG. 1 of the accompanying drawings. As seen by the projection lens, the panel therefore resembles a microfilter colour display panel as used in direct-view displays. However, the losses caused by absorption in the filters in such displays are avoided.
A related system is disclosed in EP1089115. Here angular colour separation is used with a reflective panel and the structure of the reflective pixels is modified to reflect light in the correct direction for the projection system.
These systems share some of the advantages of both the time-sequential and the three-panel architectures. No light is lost in colour filters and only one panel is used. A disadvantage is that the spatial resolution of the system is reduced by a factor of three.
Another disadvantage is that, in order to achieve a high throughput of light, the microlens arrays must have a low f-number. Each lens has a diameter equal to three colour sub-pixel widths so that, as the light valve panel size becomes smaller, the required separation between lenses and the panel pixels also becomes small. Because typical liquid crystal light valves are manufactured on a glass substrate with thickness of order 0.5 mm, there is a lower limit to the separation which can easily be achieved. For smaller separations, lenses must be integrated into the panel itself. This is possible in principle, but means that light valves must be specially manufactured for this purpose, thus increasing the cost and complexity.
Systems with angular colour separation, as described above, suffer from the problem that the resolution is reduced by a factor of three compared to the base panel.
This problem can be removed by switching the colours between the different directions rapidly. For example, in time frame 1, red light might follow the path shown by dotted lines in FIG. 1, green light the path shown by solid lines, and blue light the path shown by dashed lines. In time frame 2, red light would be represented by solid lines, green by dashed lines, and blue by dotted lines. In time frame 3, red light would be represented by dashed lines, green light by dotted lines and blue light be solid lines. The image displayed by the light valve is changed for each time frame so that a colour image is built up over the three frames.
As in a time-sequential colour projector, if the projector switches rapidly enough between these three states, the eye fuses the red, green and blue light from each pixel into a single, colour image. This type of projector design is known as ‘angular time-sequential colour’ (ATSC). The problem of reduced resolution for angular colour separation is thus removed but the problem of panel-barrier separation remains.
In all known ATSC projectors, a white light source is used and there is a mechanism which separates the white light into three colour beams travelling at different angles and which is able to switch rapidly the colours between the different directions. U.S. Pat. No. 5,969,832 discloses two different mechanisms for this purpose. In the first mechanism, a holographic optical element (HOE) causes light of different colours to travel in different directions. There are three different HOEs, each HOE giving a different mapping of colours to directions. Switching is achieved by sequentially moving the three HOEs into the active position on a moving belt. In the second mechanism, the three colours are separated by dichroic mirrors. After being reflected by these mirrors, the light is reflected by another mirror before reaching a microlens array adjacent to the light valve. Tilting this second mirror changes the angles of the red, green and blue rays and switches the colours of the pixels. A similar system is also disclosed where the lens array shifts relative to the panel to switch the colours of the pixels.
Japanese patent application number 2001223178 discloses another mechanism which depends upon switchable holographic optical elements to switch colours between angles.
U.S. Pat. No. 6,547,398 discloses two designs. In both, colours are separated by dichroic mirrors and a microlens array focuses the different colours to spots in its focal plane. A difference between these designs and others is that the microlens array is not close to the light valve. Instead, the system of colour spots is re-imaged by a double lens system onto the panel. In the first design, the microlens array shifts in its own plane to switch the colours of the pixels; in the second design, a mirror tilts as in the second mechanism of U.S. Pat. No. 5,969,832.
Re-imaging of the colour spots as disclosed in U.S. Pat. No. 6,547,398 is a solution to the problem of lens-panel separation suffered by both angular colour projectors and ATSC projectors. However, it has the disadvantage that because macroscopic lenses are used, aberrations may make it difficult to align the system. Also, because a lens at least the size of the panel is required, the microlens array must be separated from the panel by at least two panel diameters: this increases the size of the system.
All the projector designs mentioned hereinbefore (and in fact all commercially available electronic projectors) use white light sources such as high-pressure discharge lamps. However, recent advances in the technology have made LED illumination possible for low-power projectors.
The advantages of using LEDs as light sources in projectors are their small size, long lifetime, robustness and low temperature and pressure of operation. They can also be more efficient as single-colour light sources than conventional types of lamp. The possible replacement of conventional lamps by LEDs in various applications is reviewed in an article by Bergh et al, ‘The promise and challenge of solid-state lighting’, Physics Today December 2001 pp 42-47.
The known designs of LED projectors fall into three classes: projectors using microfilter panels; projectors using time-sequential colour; and three-panel projectors.
An article by Keuper et al, ‘Ultra-compact LED based image projector for portable applications’, SID 2003 Digest paper P-126 discloses three designs for LED projectors. Two of these use white LEDs with microfilter panels. Microfilter panels are the type of panel used in direct-view displays, where pixels are arranged in groups of three and each group has one pixel with a red filter, one with a green filter, and one with a blue filter.
The main advantage of this type of projector is low cost. Microfilter panels are available very cheaply because they are used in portable telephones and other portable electronic devices. Also the projector design is very simple. This type of LED projector has the disadvantage that at least two-thirds of the light is absorbed by the filters. It is therefore inefficient in its use of light. The resolution is also coarser than that of the base panel by a factor of three.
Electronic light sources such as LEDs are particularly suitable for time-sequential colour projectors for two reasons. They are efficient as sources of light of a single colour and they can be switched on and off very rapidly. By using LEDs, it is therefore possible to avoid the use of filters, which absorb two-thirds of the light in time-sequential colour projectors with white-light sources. Red LEDs are illuminated while the red image is displayed on the light valve, and similarly for the other colours.
The article by Keuper et al also discloses a time-sequential colour LED projector. Other such designs are disclosed in WO02080136, U.S. 20030133080, EP0888016, and EA01347653. This type of projector has the advantages that it is efficient in its use of light, that it uses only a single panel, and that the resolution of the base panel is maintained. Its disadvantages are the presence of flicker and colour defects if the frame rate is not high enough and the fact that the panel must run at at least three times video speed, which makes it expensive.
A third type of LED projector design is to illuminate one panel with red LEDs, one with blue, and one with green. The images from the three panels are then combined in the same way as in a conventional three-panel projector. Projectors of this type are disclosed in U.S. Pat. No. 6,224,216 and U.S. Pat. No. 6,281,949. This type of projector has the advantages that it is efficient in the use of light and that it maintains the resolution of the base panel. However, the use of three panels adds to the cost and bulk of the projector.
A different LED projector design is disclosed in Japanese application 2001371785. An array of LEDs generates a small block image and a vertical and horizontal scanner rapidly diverts the image of this block as the pattern of LED illumination is changed. A larger image is thus built up over time. This design has high light efficiency but has the disadvantage of requiring high-speed mechanical scanning apparatus, which is expensive and unreliable.
EP01024669 discloses a design for an LED illumination system for projectors which includes a reflecting surface for collimation and means for converting a large fraction of the light emitted to a single polarisation state.
V. Medvedev et al: ‘Uniform LED illuminator for miniature displays’ SPIE Proceedings vol. 3428, pp 142-153 (1998) discloses a similar reflective illuminator. G. Harbers et al: SID Microdisplay 2002, Digest of papers pp 22-25 (2002) also discloses how high-power LEDs can be used in electronic projectors.
It is well known that the human visual system makes little use of blue in perception of the fine details of a scene, for example as disclosed in J. S Wolffsohn et al, ‘Contrast is enhanced by yellow lenses because of selective reduction of short-wavelength light’, Optometry and vision science vol 77, pp 73-81 (2000), and J. K Hovis et al, ‘Physical characteristics and perceptual effects of blue-blocking lenses’, Optometry and vision science vol 66, pp 682-689 (1989).
In three-panel projectors, it is possible to take advantage of this fact by using a lower-resolution panel for the blue channel than for the red and green channels, as disclosed in R. Martin et al: ‘Detectability of reduced blue pixel count in projection displays’, Proceedings of the Society for Information Display, vol 24, pp 606-609 (1993).
This lack of sensitivity to high-frequency blue information can also be used in direct-view displays, where the arrangement of colour filters can be changed to match the characteristics of the display to those of the human visual system. This is disclosed in; WO02091348, U.S. 2002015110, U.S. application 20030128179, U.S. application 20030090581, C. H. Brown-Elliot, ‘Reducing pixel count without reducing image quality’, Information Display vol. 99(12) (1999), and T. L. Credelle et al: ‘MTF of high-resolution Pentile matrix displays’, Eurodisplay 2002 Digest, pp 159-162).
It is possible to enhance the light throughput in some types of electronic projectors and direct-view displays by integrating microlens structures into the pixels of the light valve. U.S. Pat. No. 5,682,215 discloses such a technique, and gives two methods of placing lenses into the structure of a transmissive liquid crystal display panel. These methods are: ion implantation to change the local refractive index profile of the glass substrate; and etching relief structures into the substrate which are then filled with a polymer resin of a different refractive index. U.S. Pat. No. 5,844,644 discloses an alternative scheme where the lenses are incorporated in an ‘overcoat’ layer placed over the colour filters.